1. Field of the Invention
This invention relates to apparatus for applying a glass frit to a sealing edge on the funnel portion of a picture tube.
2. Description of the Prior Art
When a picture tube or a television cathode ray tube is manufactured, a funnel portion 1 and a nearly rectangular bowl-shaped panel portion 2 are separately formed, as shown in FIG. 1. Then, a sealing edge or an end surface 1a of the funnel portion 1 is adhesively fixed to a corresponding end surface of the pannel portion 2. In effecting such adhesion, the end surface 1a of the funnel portion 1 is coated with a so-called glass frit.
There exists a coating apparatus in which the end surface 1a of the funnel portion 1 is automatically coated with the glass frit. In the existing coating apparatus, the end surface 1a of the funnel portion 1 is directed upward while the funnel portion 1 is rotated round its central axis so that the peripheral speed of the funnel portion 1 is constant with respect to an outlet opening or nozzle of a glass frit containing pot which is disposed above the end surface 1a of the funnel portion 1. The glass frit containing pot is arranged so as to be movable toward and away from the central axis of the funnel portion 1 for alignment with the end surface 1a. The glass frit is discharged onto the end surface 1a of the funnel portion 1 from the outlet opening of the glass frit containing pot at a constant rate, so that the end surface 1a of the funnel portion 1 is coated with the glass frit.
As is apparent from FIG. 2, the width d.sub.1 of end surface 1a at its corners is larger than the width d.sub.2 at the relatively straight portions of the end surface 1a on the funnel portion 1. The difference between the widths d.sub.1 and d.sub.2 is an inherent result of the process used for manufacturing the funnel portion 1. Although the difference can be reduced to a certain extent, it cannot be completely eliminated. Particularly, when the funnel portion 1 is manufactured by a spinning method, the difference between widths d.sub.1 and d.sub.2 is considerable. Therefore, each corner should be coated with more glass frit per unit length of end surface 1a than each straight portion of the latter.
Since, in the existing apparatus, the funnel portion 1 is rotated about its central axis at a constant relative speed between the end surface 1a and the outlet opening of the glass frit containing pot and the glass frit is discharged from the outlet opening of the glass frit containing pot at a constant rate, the coated layer of the glass frit is too thin at the corners of the end surface 1a of the funnel portion 1, or it is too thick at the straight portions of the end surface 1a. Heretofore, the thickness of the coated layer has been manually evened with a knife or a spatula. Such manual operation requires considerable skill. Further, the powder of the glass frit often scatters, and considerable investment is required for the prevention of polution of the environment thereby.